The development of continuous dye lasers now provide the scientific community with a source of tunable coherent radiation at visible wavelengths, and such lasers and their associated systems have undergone steady improvements in recent years both in output beam quality and narrowness and stability of the output beam frequency. Continuous dye lasers employ a flowing liquid, typically in the form of a fla, free-flowing jet stream, as an optically efficient means of coupling light into and out of the dye medium. The dye medium, in the form of this flowing liquid stream, is capable of withstanding the high power densities of the focused pump laser beam. However, flowing liquids such as the dye may contain bubbles and also may vary in thickness in response to fluctuations in the pressure head driving the flow, despite elaborate precautions in the form of pressure regulators and filters. These perturbations cause amplitude and frequency perturbations of the dye laser output, since even a tiny bubble can block the focal region in the dye and momentarily terminate the lasing function. The frequency perturbation results from the relationship expressing the frequency .nu., which, for the n.sup.th lasing mode is given by EQU .nu..sub.n =n(c/2D) (Eq. 1)
where n is a large integer, c is the speed of light and D is the optical path length, including the dye stream thickness, between the end mirrors of the laser cavity. By equation 1 a thickness change of .lambda./2 (where .lambda. is the wavelength of light, between 4 and 8.times.10.sup.-7 meters for visible wavelengths) produces a frequency change of c/2D or 300 MHz for a typical cavity length D of 0.5 meters. In such a system the integer n is of the order n=2D/.lambda.=2.times.10.sup.6. Since currently available dye lasers are capable of frequency stabilities of .+-.1 MHz or less, they are thus subject to very minute optical path length changes. Since temperature and air pressure variations may also cause variations in the length of the optical cavity, such variations may also produce output frequency changes of this order of magnitude. The presence of these problems has led to the development of means, such as servo "locking" of the dye laser output frequency to improve the frequency stability. Such arrangements have generally sought to lock the dye laser output frequency to equal that of some resonance of a stabilized reference interferometer (as in the manner disclosed by Barger, Sorem and Hall in "Frequency Stabilization of a CW Dye Laser", Applied Physics Letters 22, pp. 573-75, June 1, 1973, and by Barger, West and English in "Fast Frequency Stabilization of a CW Dye Laser, Applied Physics Letters 27, pp. 31-33, July 1975) or by use of atomic or molecular spectral features. This technique is used in particular with single frequency dye lasers, where a succession of increasingly narrow bandwidth filters is placed in the dye laser cavity to restrict oscillation to a single cavity mode, which represents a single value of the integer n.
Where a stable reference interferometer is used, its resonant frequencies are given by the relationship EQU .nu..sub.m =m(c/2L) (Eq. 2)
where m is an integer, c is the speed of light as noted above, and L is the optical pathlength between the interferometer mirrors. While the prior art has incorporated such an approach, it has been concerned only with conditions around the specific, narrowly defined frequency lock point, primarily to control and eliminate the effects of dye frequency jitter or noise. Such prior art systems provide a stable output frequency as long as the system remains continuously locked to the servo frequency lock point, but have had no protection against loss of lock such as often occurs with the passage of a bubble or minor pressure variation through the dye stream. In these prior art systems the occurrence of such perturbations has generally limited the time periods of constant output frequency operation of the dye laser to a maximum on the order of ten minutes of continuous operation. Such prior art apparatus has not been capable of automatically and rapidly reacquiring the preselected locked dye laser output frequency when subjected to large momentary laser cavity length changes or momentary losses of lasing.